yes, when dealing with an equation, if you do one thing to one side of the equation, you must do the same to the other side.
Since we had
108 + x = 20 * 6
we want to just have x on the left side, so we subtract 108 from each side.
108 + x - 108 = 20 * 6 - 108
the 108s on the left cancel, like we wanted.